A commonly used piezoelectric ceramics is an ABO3-type perovskite oxide such as lead titanate zirconate (hereinafter, referred to as “PZT”).
However, it is considered that PZT, which contains lead as an A-site element, may cause environmental problems. Therefore, a piezoelectric ceramics with a lead-free perovskite-type oxide has been desired.
Barium titanate is known as a material for a lead-free perovskite-type piezoelectric ceramics. Patent Literature 1 discloses barium titanate prepared by a resistance heating/two-step sintering technique. The patent literature describes that a ceramics with excellent piezoelectric property can be obtained when nano-sized barium titanate powder is sintered by the two-step sintering technique. However, the ceramics obtained by the two-step sintering technique is not suitable for use in a resonance device because it has a small mechanical quality factor and low high-temperature durability.
Further, Patent Literature 2 discloses a ceramics obtained by replacing part of a barium site in barium titanate with calcium and further adding manganese, iron, or copper. The patent literature describes that the ceramics has an excellent mechanical quality factor by virtue of manganese, iron, or copper. However, an increase in calcium content shifts the temperature of crystal phase transition to about −50° C., resulting in a remarkable decrease in piezoelectric performance.
Further, it is well known in the art that an increase in amount of manganese added leads to precipitation of manganese oxide (MnOx) outside crystal particles of barium titanate. The manganese oxide lacks properties of a dielectric substance, and hence causes a decrease in piezoelectric performance of a ceramics. Further, the manganese oxide causes a decrease in mechanical quality factor because the valence of manganese is unstable.
In other words, a barium titanate-based piezoelectric ceramics is expected to have both of satisfactory piezoelectric performance and a high mechanical quality factor.